In offshore drilling, three legged triangular hull constructions are quite common. These hull constructions have serious shortcomings because the major bulkheads and scantlings are arranged longitudinally and transversely with respect to the hull centerline. The procedure of framing hulls longitudinally and transversely is as old as shipbuilding itself and is the best method for framing ships and rectangular barges. However, when applied to triangular hulls the following problems are encountered:
A. Most major bulkheads and other structural members are of varying lengths and sizes with considerable variation of joint details where the members attach to each other.
B. Modular construction, which is widely used as a cost reducing method, is extremely difficult since there are few logical points at which the hull can be broken down into modules. In the event modular division is accomplished, the modules are of different sizes with varying scantlings.
C. The distribution of shear and moments throughout the hull girder is quite inefficient, due to the fact that these forces act along lines connecting the three legs. Thus the primary structures are, for the most part, running at a 30.degree. angle with relation to the natural line of action of the forces acting on the hull. The results are that shear distribution among bulkheads is such that some have to be specially reinforced, and in some cases the addition of more bulkheads is required, while some carry practically no shear load. Plate stiffeners at some locations are skewed with relation to the bending moment acting on the hull such that their cross sectional areas cannot be included in overall hull girder strength, thus requiring thicker plating.
D. Compartments within the hull are of varying sizes and shapes. This poses difficulty in locating equipment within the hull and results in considerable unusable spaces. Watertight subdivision for stability purposes is very inefficient with additional bulkheads sometimes being added strictly for the purpose of obtaining proper satertight subdivison.
The hull of the present invention overcomes the problems noted above. By abandoning the conventional methods of framing and instead framing the hull in a direction consistent with the forces acting upon it, a new and greatly improved hull is realized which results in considerable weight savings and savings in labor cost during fabrication. It provides a modular construction consisting of a minimum number of modules of a minimum number of different sizes with modules of the same size being of identical construction. Further, the members comprising each module are of similar sizes and lengths. Special joint details are practically eliminated.
Bulkheads and plate stiffeners are arranged in the mot efficient manner for resistance of shear and moments acting on the hull girder. The bulkheads arrangement also represents the optimum watertight subdivision of the hull. Less structural material is required in the hull because the loads are distributed uniformly throughout the structural members, thus avoiding unduely loaded members that must be reinforced. In the most highly stresses parts, minimum reinforcing over the regular scantling requirements is added.
Size and configuration of machinery deck compartments accommodate placement of equipment in the most efficient manner resulting in the maximum utilization of all available space.